Electron microscopes have spatial resolution exceeding the optical limit and enable observation of fine structures on the order of nm to pm and analysis of compositions. Thus, electron microscopes are widely used in the fields of engineering such as materials, physics, medical science, biology, electricity and mechanics. A type of electron microscope with which a sample surface can be easily observed is a scanning electron microscope (SEM). Recently, it is desired that SEMs enable observation of the top surface of a sample or observation of a light element substance such as carbon-based compounds. Such observation can be achieved by “low acceleration observation” in which the energy of the electron beam that reaches the sample is reduced to around 5 keV or less. However, the spatial resolution during low acceleration observation has a problem because the spatial resolution deteriorates due to the chromatic aberration of the objective lens. The chromatic aberration can be reduced by reducing the variation in the energy of the electron beam. Thus, in order to achieve high resolution also for low acceleration observation, it is required to reduce the variation in the energy of the electron beam, that is, a monochromatic electron beam is necessary.
The monochromaticity of an electron beam is determined by the properties of the electron source that emits the electron beam. Thus, by selecting the optimal electron source, the resolution during low acceleration observation can be increased. There are mainly three kinds of electron source that are currently used in practical applications, and the three are thermionic electron sources, cold field emission (CFE) electron sources and Schottky electron sources.
Thermionic electron sources are electron sources which emit thermoelectrons by continuously heating filaments made of tungsten (called W below) to around 2500° C. When W is heated, the energy of electrons in W becomes higher than the Fermi level. The energy of some electrons exceeds the work function, and the electrons are emitted into the vacuum. The variation in the energy of an electron beam is represented by the full width at half maximum of the energy distribution (called an energy width below). The energy width of a thermionic electron source is around 3 eV to 4 eV, which is the greatest among the three types of electron sources, and it is difficult to achieve high resolution during low acceleration observation with this kind of electron source.
Reasons why the energy width of the electron beam emitted from a thermionic electron source is great are that electrons have a variation corresponding to the thermal energy when the electrons are heated and that the electrons with the variation are directly emitted. As the heating temperature increases, the energy width becomes greater. Thus, a thermionic electron source in which the heating temperature is decreased using a material other than W and a thermionic electron source which has an electron emitter with a tip narrowed to the order of μm have been put into practice. However, the energy widths of both sources are still great and are around 2 eV or more.
The principal of electron emission from CFE electron sources is different from that from thermionic electron sources, and CFE electron sources are electron sources which emit electrons through the quantum tunneling effect. When an extraction voltage is applied using a W needle with a sharpened tip as the negative electrode, the field concentrates at the tip, and electrons are emitted. This phenomenon is called field emission. The field at the tip is at minas several volts per nanometer. Through field emission, the electrons in W which have energy around the Fermi level mainly slip through the potential barrier and are emitted into the vacuum. The energy width of the electron beam emitted from a CFE electron source is narrow and is 0.3 eV to 0.4 eV, which is the narrowest among the three types of electron sources. In order to achieve high resolution during low acceleration observation, CFE electron sources are the optimal electron sources. When a CFE electron source is heated during its use, the number of electrons with energy higher than the Fermi level increases, and the energy width of the electron beam slightly increases. Thus, CFE electron sources are generally used at room temperature or at a lower temperature. This is why these sources are called cold sources.
The tip of the W needle of a CFE electron source is sharpened to a radius of curvature of around 50 nm to 150 nm to cause field concentration. The tip is very fine, and thus the surface of the tip has a structure of a combination of crystal faces. Moreover, the alignment of the crystal faces reflects the crystal structure and is regular and constant. Each crystal face has its own work function, and the work function is a value reflecting the crystal structure on the surface of the fine tip. As the work function reduces, the current released through field emission becomes higher. The stability and the brightness of an electron beam also differ with the crystal face. The electron beam emitted from the {310} plane with a work function of 4.3 eV and from the {111} plane with a work function of 4.5 eV is mainly used as probe current in W.
The shape of the tip, the sizes of the crystal faces and the work function vary with the material used for the electron source. Thus, the characteristics of the electron beam differ with the material, and the optimal electron-emitting face is also different. CFE electron sources made of various materials other than W have been studied so far, but none of them have been put into practice. For example, NPL 1 describes a CFE electron source using cerium hexaboride (called CeB6 below), which is expected to achieve a small work function.
Like thermionic electron sources, Schottky electron sources emit thermoelectrons, but the surface with a decreased work function is characteristic of Schottky electron sources. A W <100> single crystal needle coated with ZrO is used for the electron sources, and the tip has a radius of curvature of around 0.5 μm to 1 μm, which is slightly larger than that of CFE electron sources. When this kind of electron source is continuously heated to 1400° C. to 1600° C., the ZrO coating specifically acts on the {100} plane and decreases the work function. Moreover, the Schottky effects are caused when a field is applied to the tip, and the effective work function further decreases. As a result, the work function of the {100} plane becomes around 2.8 eV. The energy width of the electron beam emitted from this plane is narrower than that of thermionic electron sources and is around 0.6 eV to 1 eV. This energy width is in the middle among the three types of electron sources. Schottky electron sources using a coating material other than ZrO are also studied, but most of them have not been put into practice.